1. Field of the Invention
Apparatuses consistent with the present invention relate to an ultra wideband filter for filtering an input voltage, and more particularly, to an ultra wideband filter for filtering an input voltage using a pair of cross-coupled nMOS and pMOS transistors.
2. Description of the Related Art
Generally, a communication system uses a frequency of a certain band to transmit data. A communication system that uses a relatively wide frequency band is referred to as an ultra wideband (UWB) communication system. The ultra wideband communication system transmits and receives data using a band of several hundreds of MHz or greater so as to transmit and receive a large amount of data per unit time. In this respect, the ultra wideband communication system requires an ultra wideband filter that filters an ultra wideband signal.
Meanwhile, an active R-C filter, a Gm-C filter, and other filters have been used to filter an analog signal.
FIG. 1 is a circuit diagram illustrating a conventional active R-C filter. Referring to FIG. 1, the conventional active R-C filter includes a resistor R1, a capacitor, and a Miller integrator consisting of an operational amplifier. A transfer function of a frequency in the circuit of FIG. 1 is obtained by Equation (1).
                              T          ⁡                      (            s            )                          =                                                            V                o                            ⁡                              (                s                )                                                                    V                i                            ⁡                              (                s                )                                              =                                    -                              1                                                                            R                      1                                                              R                      2                                                        +                                                            sC                      2                                        ⁢                                          R                      1                                                                                            =                          -                                                1                                                            C                      2                                        ⁢                                          R                      1                                                                                        s                  +                                      1                                                                  C                        2                                            ⁢                                              R                        2                                                                                                                                                    Equation        ⁢                                  ⁢                  (          1          )                    
However, since the active R-C filter uses an operational amplifier having high voltage gain, the available frequency band of the active R-C filter is limited to 100 MHz or less. Therefore, the active R-C filter is not suitable for an ultra wideband communication system. Also, a problem arises in that the size of the filter increases due to a large size of the operational amplifier.
Meanwhile, FIG. 2 is a circuit diagram illustrating a conventional. Gm-C filter. Referring to FIG. 2, the conventional Gm-C filter includes a plurality of transconductors T1 and T2, and a plurality of capacitors CX and CA. If an input signal is applied to a non-inverting terminal of the first transconductor T1, a predetermined output signal having a varied amplitude and phase of the input signal is output through the second transconductor T2. In the circuit of FIG. 2, the input signal-to-the output signal ratio and a transfer function can be obtained by Equation (2).
                              H          ⁡                      (            S            )                          =                                                            v                o                            ⁡                              (                S                )                                                                    v                i                            ⁡                              (                S                )                                              =                                                                      (                                                            C                      X                                                                                      C                        A                                            +                                              C                        X                                                                              )                                ⁢                S                            +                              (                                                      Gm                    ⁢                                                                                  ⁢                    1                                                                              C                      A                                        +                                          C                      X                                                                      )                                                    S              +                              (                                                      Gm                    ⁢                                                                                  ⁢                    2                                                                              C                      A                                        +                                          C                      X                                                                      )                                                                        Equation        ⁢                                  ⁢                  (          2          )                    
In Equation (2), Gm1 and Gm2 represent transconductances of the first and second transconductors, respectively, and CX and CA represent capacitance of each capacitor.
The Gm-C filter has a problem that it is not suitable for the ultra wideband communication system because of its limited frequency band. Also, to design the Gm-C filter, elements such as a plurality of transistors and capacitors are required. A tuning circuit is additionally required to obtain desired characteristics because the Gm-C filter is susceptible to process variation, temperature variation, and so on. For this reason, problems arise in that power consumption increases and the size of the filter increases. Moreover, it is difficult to exactly obtain a filter specification of a desired design due to capacitance of respective elements of the Gm-C filter.